I don't have any evidence about adverse effects not to use HEPES-saline for injection in animal model(s), just be sure to filter sterilize your buffer before usage.
One of the design criteria for Good-buffers (doi:10.1021/bi00866a011) was biological inertia. Many cell cultures tolerate up to 50 mM HEPES, so a small volume of 20 mM should have no effect. I assume that you have permission for your experiment from your local animal use committee? You need to inform them of any changes from the approved protocol.
I don't know about the biological compatibility of HEPES, but it seems to me advisable to use a buffer that would be isotonic, i.e. with 120 mM NaCl in addition to HEPES